This is a 371 application of PCT/GB99/00560 filed Feb. 23,1999.
This application relates to the evaporation of samples comprising solid material dissolved or suspended in a liquid in a vacuum, and to improved methods for determining and controlling the temperature of such samples while they are evaporating. It is particularly applicable to monitoring samples in and control of centrifugal evaporators.
In centrifugal evaporators samples are usually held in glass or plastic tubes or, sometimes, in a large number of small wells in plastic blocks. The sample holders are spun so as to arrange for a considerable centrifugal force to be applied to them in a direction which forces the liquid to the lower part of the sample tubes to prevent any frothing or spitting of the liquid out of the sample tubes when a vacuum is applied. The spinning samples are held in a vacuum-tight chamber (referred to henceforth as xe2x80x9cchamberxe2x80x9d) which is connected to a vacuum pumping device.
Evaporators of this type are well known and many types are available commercially.
They all suffer from the difficulty of providing the latent heat of evaporation to the samples to allow evaporation at reasonably high speed without allowing the samples to reach temperatures which could damage or destroy the samples which are often thermolabile.
No heat can be provided by conduction because the samples are held in a vacuum but microwaves or radiant heat from a high temperature source (500xc2x0-3000xc2x0 C.) can be used to provide the heat required for evaporation. Radiation from a low temperature source, eg the chamber walls at 40xc2x0 C., is frequently used but it cannot provide sufficient heat for rapid evaporation of anything other than small samples of highly volatile solvents. The use of radiant heat and of microwaves is known. Microwaves can cause damage to some samples and in the known embodiments that use radiant heat, the heat has been applied in a manner in which samples are not heated uniformly so that some samples can be dry whilst others are still liquid. This causes overheating of the dry samples if the heat is maintained long enough to dry the last samples.
According to one aspect of the invention in a method of evaporating liquid samples contained in at least some of a plurality of individual sample holders which are mounted within a chamber and rotated during the evaporation process so that centrifugal force is exerted on liquid contained therein during the evaporation process, and wherein heat is supplied to the sample holders to heat the liquid therein whilst a pressure below atmospheric is maintained in the chamber in manner known per se, a temperature sensing device is located in or adjacent at least one of the sample holders to sense the temperature therein at least during the evaporation process and to generate an electrical data signal which is proportional to the sensed temperature, and a signal path is provided to convey the temperature data signal to electronic data signal processing means.
Typically the data signal processing means is located at the centre of rotation of the plurality of sample holders.
Conveniently the processing means converts the output of the sensor into a suitable form for transmission to an external receiver.
The processing means may convert the sensor output signals into digital or analogue signals by which a carrier signal is modulated to effect the said transmission.
Conveniently the transmitted signal constitutes a radio signal. This for example may be transmitted to a receiver located externally of the housing by means of an antenna which is located externally of the housing and is connected to the signal processing means by means of a conductor which passes through the housing wall via an insulating seal serving as a lead through.
Where the chamber wall does not readily transmit, or significantly attenuates radio signals, the radio signals from the signal processing means may be received by a stationary radio receiver located within the chamber and conveyed either as radio signals or after demodulation as data signals indicative of the temperature of the sensor, via a conductive path which extends sealingly through and is insulated from the chamber wall. Typically the signals are conveyed through the chamber wall as radio signals, for demodulation to produce the said data signals outside the chamber.
The carrier signal may be a beam of light and the modulation is such as to modulate the intensity of the beam. In this event the light signals may be transmitted through a window which is light transmitting and which forms an integral part of the housing wall, to enable the modulated light beam to pass to a stationary light responsive device located externally of the housing and which is adapted to convert the received light signals into data signals indicative of the temperature of the sensor.
Whatever form the carrier signal and transmission system takes, the data signal may be employed to drive an indicator which is calibrated to indicate sample temperature.
Likewise the data signal may be employed to control the source of heat heating the sample holders in the chamber.
Power for the processing means may be derived from a battery located within a housing within which the processing means is also located. The battery may be connected to the processing means by the closing of a motion sensitive switch which closes when the chamber rotates, and is disconnected therefrom by the opening of the switch when the chamber ceases to rotate.
Alternatively power for the processing means may be transmitted from a source located external to the housing to a receptor located within the housing which is connected to the processing means.
In a further alternative arrangement, power for the processing means may be supplied thereto from an external power source by means of a rotational electrical connection. Such a connection may comprise slip rings and conductive elements in contact therewith.
Preferably any rotational electrical connection is separated from vapours in the chamber by being located outside the chamber, or inside the signal processing means housing, and seals are provided around conductors leading between the signal processing means and the external electrical connection where they pass through the wall of the chamber or the housing.
Typically at least one of the conductors leading between the processing means and any external rotational electrical connection, extends through the hollow interior of a drive shaft which itself extends through a seal in the chamber wall and serves to rotate both the sample holders and the said housing within the chamber.
The drive shaft may itself be electrically conductive and serves as one of the conductive paths for the power to the signal processing means.
Preferably material from which the housing is constructed, is non-conductive as well as being inert in the presence of the vapours given off during the evaporation process.
A preferred material for the housing is polypropylene.
In a further arrangement relating to the supply of power to the signal processing means in the housing, power therefor is generated in a winding which rotates with the housing relative to a stationary magnetic flux.
The winding in or on the housing may be wound on soft magnetic material such as is employed to make transformer laminations. It may be mounted on a central spindle which rotates the sample holder in the chamber.
Typically the magnetic flux is produced by at least one permanent magnet which comes into close proximity with the winding during each rotation of the sample holder, and is located either inside the chamber and the winding is in or on or close to the housing, or is located outside the chamber and the winding is rotated around the interior of the chamber close to the wall thereof.
The sensor may be sheathed in an impervious inert material so that it will not contaminate the sample or suffer corrosion, and may for example comprise a thermocouple.
The sample holders are typically rotated at a speed of between 500-3000 rpm depending on the g-force required to be imposed on the sample and the radius at which the samples are rotated.
According to another aspect of the invention, centrifugal evaporating apparatus comprises a vacuum chamber, a plurality of sample holders for containing liquid samples to be evaporated, which are located therein for rotation about a generally vertical axis, heating means for heating the sample holders and therefore the liquid samples therein, temperature sensitive probe means located in or adjacent at least one of the sample holders, signal path means for conveying electrical signals from the probe means to a signal processing means located within the chamber, a transmitting device also within the chamber for transmitting signals to a receiver outside the chamber, signals from the signal processing means being employed to modulate the transmitted signals so that when decoded by the remote receiver, the latter will provide a signal containing information about the temperature of the probe.
Typically the transmitting device transmits radio signals which the receiver is adapted to receive.
The signal processing means is preferably housed in a leak-tight housing to protect the electronic components making up the processing means from pressure fluctuations and from the vapours arising from evaporation in the chamber.
The apparatus may comprise an indicating means calibrated to indicate temperature, and controlled by signals decoded by the said remote radio receiver, to indicate the temperature of the probe.
The heating means in the chamber may be such that its heat output is controlled by the magnitude of an electrical current, and current controlling means is provided adapted to control the said electric current to the heating means, and the decoding signal from the remote radio receiver containing the temperature information is employed to control the current controlling means and thereby the heat output from the heating means and in turn the temperature to which the probe and therefore the liquid samples are permitted to rise.
Power for the signal processing means may be provided from a battery which may be located in or externally of the housing containing the said processing means.
Alternatively power for the signal processing means may be provided from a power supply which remains stationary and is external to the housing containing the signal processing means and a path is provided between the power supply and the processing means for conveying power thereto as the housing rotates relative to the power supply.
Alternatively power for the signal processing means may be derived from relative movement between the stationary magnetic field of the flux which links with winding which rotates relative to the magnetic flux thereby to induce a current in the winding which is available to power the signal processing means.
The winding may be on a spindle on which the housing is mounted, and which rotates both the housing and the sample holders within the chamber, or on, or in the housing itself, or may be rotated by the spindle around a circular path inside the chamber close to the wall thereof, and the magnetic flux passes through the chamber wall to link with the winding as it travels around the interior of the chamber.
While the samples are evaporating, large amounts of heat can be coupled into the samples without danger of overheating, by maintaining the pressure in the vacuum chamber at a low level.
In most cases the sample liquid is made to boil and it will boil at a temperature determined by the pressure at the boiling surface of the sample.
Evaporative cooling therefore controls the temperature to a safe level provided the vacuum pumping system maintains the chamber pressure at a sufficiently low pressure, but as soon as the samples are dry, evaporative cooling stops and sample temperature can rise rapidly to unacceptable levels if the heat input is allowed to continue.
It is difficult to measure the sample temperature during an evaporation run because the sample is rotating in a vacuum chamber during the process, and the invention assists in solving this problem.
Thus measurement can be achieved as desired, by inserting a temperature sensor into one of the samples and connecting it to a suitable electronic processor and transmitter, preferably located at the centre of rotation of the sample holder.
This sensor can be a thin thermocouple sheathed in an impervious inert material such as PTFE so that it will not contaminate the sample or suffer corrosion.
The processor preferably amplifies and converts the output of the thermocouple to a digital or analogue signal, which is transmitted through the container, into the vacuum chamber, and is picked up by an external receiver.
Transmission of the thermocouple signal can be by means of a modulated electromagnetic signal or by a suitably modulated light or infra-red beam or by any other means which will allow external detection of the signal, the only requirement being that the chosen radiation can be transmitted through the container, and possibly also the chamber wall.
Other heat sensors can be used in place of a thermocouple, but a thermocouple is convenient because it can be made small.
Power can be supplied to the electronic circuitry by making the rotating drive shaft hollow and feeding conductors through the hollow interior and through suitable seals to the inside of the electronic container. The grounded outer part of the drive shaft may again be taken into the housing by means of a suitable seal. Power would still have to be fed to the conductors by means of slip rings or brushes, but this may be easier than methods which necessitate a rotating seal.
In cases where it is undesirable to place a sensor in all actual sample, it can often be arranged that a tube is left empty adjacent to other sample tubes, and if this is filled to the same level as the sample tubes, with a similar inert liquid, and subjected to the same heating, a temperature sensor placed in such a tube can give a good approximation to the actual temperature of the samples.
If such a blank tube is not available, an alternative location must be found in which to place the sensor close to the sample tubes in a position where it experiences temperatures close to those of the samples. For instance, in sample holders comprising a plurality of holes in a block of plastics or metallic material eg, aluminum, in which samples are placed, a less accurate but nevertheless reasonable approximation to sample temperature can be obtained by placing the sensor in a separate hole in the same block in which the sample retaining holes are formed.
Alternative Approach
When the sample liquid is evaporating, the pressure in the evaporator chamber approximates to the vapour pressure of the liquid or liquids in the chamber at the temperature of the liquid or liquids concerned and the chamber pressure can therefore be used as an indication of sample temperature.
The invention therefore also provides a method of determining the temperature of evaporating liquid samples contained or comprising at least one rotatable component and contained in at least some of a plurality of individual sample holders which are mounted within a chamber and rotated during the evaporation process so that centrifugal force is exerted on volatile liquid contained therein, and wherein heat is supplied to the sample holders to heat the volatile liquid therein whilst a pressure below atmospheric is maintained in the chamber in manner known per se, which is characterised by the location of a pressure sensing device in the chamber, sensing the pressure therein at least during the evaporation process, generating an electrical pressure data signal which is proportional to the sensed pressure, conveying along a signal path the pressure data signal to electronic data signal processing means which is programmed inter alia with information relating to the volatile component or components present in the samples, to convert the pressure data signal to a temperature value equal to that which equates to the measured vapour pressure for the known volatile component or components present.
The data signal processing preferably includes the step of addressing a look-up table containing temperature and pressure values for different liquids, and the further step of identifying to the signal processing means the volatile component or components present in the samples.
In addition or alternatively the data signal processing means may include an algorithm, and memory means, and the method involves the step of storing numerical values for insertion in the algorithm depending on the volatile component or components present in the samples and the pressure determined by the pressure sensing device, to enable computation of the temperature to which the sample must have been raised for the particular pressure to be observed in the chamber given the presence of the volatile component(s) concerned.
The invention also provides a method of controlling the temperature to which the samples in a centrifugal evaporator are heated by heating means within a pressure chamber containing the samples, wherein the pressure within the chamber is determined by pressure sensing means, a data signal is generated proportional to the pressure, and energy to a heating means located within the chamber for heating the samples is controlled in response to the value of the pressure signal. The temperature determination may be as described in the preceding paragraphs, by computing temperature from the pressure measurement in combination with the identity of the volatile component or components in the samples.
The invention also provides a method of supplying heat to liquid samples in a pressure vessel in a centrifugal evaporator which are contained in a plurality of tubular containers which during centrifuging in the pressure vessel are swung from a generally vertical condition to a generally horizontal condition under the influence of increasing centrifuge forces so that centrifugal forces are exerted on the liquid in the tubular containers as the pressure within the chamber is reduced, wherein radiant heat is directed towards the closed ends of the tubular containers whilst in their generally horizontal condition thereby to achieve more uniform heating of the liquid samples.
There is also provided another method of heating liquid samples contained in a centrifugal evaporator, wherein the samples are contained in a fixed regular array and more of the heat is directed to samples located in the central region of the array than is directed to samples around the periphery of the array.
In any of the aforementioned methods or apparatus, the heat source may comprise a source of infra-red radiation.
Where the heat is infra-red radiation, a heat absorbing screen is preferably located between the source of infra-red heat and the samples, having a plurality of radiation conductive regions therein, each conductive region aligning with the position of one of the samples in the array of samples, and the thermal transmissivity of the regions increase towards the centre of the array so that samples located in the central region of the array receive more radiation per unit time than those in peripheral regions of the array.
In any of the methods and/or apparatus described herein, the samples may be contained in an array of tubes, bottles or vials held in holders which uniformly swing upwardly from a vertical position to a generally horizontal position during rotation of a platform on which they are mounted, or in wells in a microtitre plate which can also be swung upwardly as aforesaid.
In any of the methods and/or apparatus described herein, the source of heating may be situated at one radial position relative to the axis of rotation of the sample containers, and each sample is subjected to radiant heat energy as it passes the source of heat during its rotation around the said axis of rotation, or alternatively the source of heat may extend around an arcuate path extending around some or all of the circular path of the samples, so that the exposure of each sample to radiant heat is for a greater percentage of its circular path than if the heat source is located at only one point around their path.
The invention also lies in a method of controlling the heating of liquid samples in a plurality of individual sample holders in a centrifugal evaporator wherein the samples are mounted for rotation within a vacuum chamber, the pressure of which is reduced during the evaporation process, wherein heating means is located in the chamber for heating the samples and control means is provided for controlling power to the heating means, and temperature signals from one or more probes, or a signal from a pressure sensor which detects the pressure within the chamber, are supplied to the heating control means, which is programmed to provide a high heat input during early stages of the evaporation process and smaller heat input during later stages of the process as the liquid within the samples is evaporated. In this way excessive overheating of dry material left within a sample holder after evaporation is complete and which is no longer subjected to the cooling effect of evaporating liquid within the sample, can be avoided.
The invention also lies in a method of controlling the heating of-samples within a centrifugal evaporator wherein the samples are contained within a pressure vessel which is progressively evacuated by a vacuum pump so as to assist in the evaporation of the liquid from the samples, wherein a vapour condenser is employed to increase the pumping speed to protect the vacuum pump from vapour emitted during the evaporation process and wherein there is provided means for measuring vapour flow rate, and method of control involves controlling the energy to the heater in response to a signal derived from the flow rate measurement such that as the flow rate decreases, the heating energy is decreased, and as the vapour flow rate approaches zero, indicating that the samples are all dry, the heat energy is shut off.
The invention also lies in a method of supplying heat to a plurality of samples in each of a plurality of sample plates, each plate containing a plurality of wells or other liquid containing devices, each capable of containing one liquid sample, wherein the. sample plates are supported on trays of material having a high thermal conductivity, and the trays themselves are supported within, and have a good thermal path to and from, a support frame also formed from high thermal conductivity material, and heat is supplied to the support frame from which it is conducted by the thermal path to the trays and thereby to the samples plates and samples contained therein. The heating may be effected by infrared radiation directed towards the said support frame.
The invention also lies in apparatus for supporting microtitre plates, each containing a plurality of liquid samples, for evaporation in a centrifugal evaporator, wherein heat is supplied to the microtitre plates via a supporting frame and via trays extending thereacross on which the microtitre plates are located, the trays and the frame being formed from a material having a high thermal conductivity, and wherein the region of each tray on which a microtitre plate is located, is formed with art upstanding region defining a platform adapted to engage a recessed underside of the microtitre plate located thereon, which would otherwise be spaced from the surface of the tray, thereby to improve the transfer of heat between the tray and the plate.
Typically the frame and the trays are formed from aluminum or copper.